Update the Spectre.Console.Cli documentation with CancellationToken

Now that #1911 is merged, all `Execute` methods of commands have a new CancellationToken parameter. Update the documentation to reflect that.

New documentation shall still be written with an example on how to pass the top-level CancellationToken to the app.Run(Async) method.

docs/input/cli/getting-started.md

@@ -33,7 +33,7 @@ internal sealed class FileSizeCommand : Command<FileSizeCommand.Settings>
         public bool IncludeHidden { get; init; }
     }
 
-    public override int Execute([NotNull] CommandContext context, [NotNull] Settings settings)
+    public override int Execute(CommandContext context, Settings settings, CancellationToken cancellationToken)
     {
         var searchOptions = new EnumerationOptions
         {